衰すい退たい期き

• 全ぜん盛せい期き

  全盛期 is the period of peak strength or popularity, while 衰退期 is the later period of decline.

• 最さい盛せい期き

  衰退期 refers to a period of decline, the opposite of 最盛期, which is the period of peak prosperity or influence.

Compound of 衰退 (decline) + 期 (period). The exact historical derivation is uncertain, but the meaning is transparent from its components.